Travelling wave tubes



July 11, 1961 A. LERBS El'AL 2,992,354

TRAVELLING WAVE TUBES Filed Feb. 17, 1955 b b L 1 4s 6 & \w

' I FIG.6 2 1 2 41 10 w \fi\ 3 Sheets-Sheet 1 July 11, 1961 A. LERBSEIAL 2,992,354

TRAVELLING WAVE TUBES Filed Feb. 17, 1955 S Sheets-Sheet 2 .F 10 I FIG.9

FIG-H men to another.

United States Patent v TRAVELLING WAVE TUBES Alfred Lerbs and DanielRevel-din, Paris, France, as-

s1 gnors to Compagnie' Generale de Telegraphic Sans F11, a-corporationof France b Filed Feb.'17, 1955, Sen-No. 488,751 "Claims priority,application France Mar. 4, 1954 26 Claims. (Cl. 315--3.5}

The present invention relates to travelling wave tubes of themagnetrontype, and more particularly to a system of electrodes forhighpower tubes of this type.

It is diflicult to construct travelling wave magnetron tubes whose peakpower orcontinuous operating power may attain 100 kw. or more. In orderto obtain such powers with tubes of this type, it is necessary to haveelectron optical systems capable of emitting very concentrated beams,having high perveance. The 'perveance is given by the expression J/ Vwhere J expresses the beam current in-amperes and V the acceleratingvoltage of the beam in volts. This perveance must be capable ofattaining in respect of 100 kw. tubes, values of the order of 10- Ibeing of the order of several amperes.

Now, in thepr'esent stated the art, it-is difficult to obtain cathodeshaving not only asurface which is sufiiciently small to emitconcentrated beams, but also a sutficient density of emission to emitsuch intense currents.

might materially diminish the -H.-'F. energy output of the tube.

Two cases may be encountered. 'In the first "case, the

impact of the electrons on the negative electrode causes no secondaryelectron emission. The electrons inquestion are lost and nolongerparticipate in t-the 'interaction mechanism between the -H.-F. wave andelectrons.

In the second case, the impact of these electrons causes a secondaryemission. rejoin the beam and may eventually intensify the energyexchange. But it is clear that such a secondary emission which takesplace over the entire surface of the electrode, depends to a largeextent on the condition of the surface of the negative electrode.Experience has shown that its main eifect is to render thecharacteristics er tubes of the same series very variable from onespeci- Theincrease in power that it might atford owing to the increasein the beam current is very variable. Moreover, such a secondaryemission increases the beam current mainly'in the vicinity of thecollector electrode. The secondary electrons do not have time to impartenergy to the wave before being'captured by the collector. The currentofthe latter, which causes aloss of energy, is therefore increasedwithout any real benefit.

The present invention has for its main object to provide high-powertravelling wave magnetron'tubes having characteristics which may bemaintained in the mass production of these tubes.

According to the invention, the negativeelectrode or these tubescomprises two parts. The surface *of "the first 1 part adjacent thecathode may include -a covering capable of causing a high "secondaryemission while'the surface -of *"the second part adjacent "the collectoris pro- The secondary electrons 2,992,354 Patented July 11, 1961 videdwith means which prevents any secondary emisstem.

The present invention will be better understood from the ensuingdescription with reference to the accompanying drawings in which:

FIG. 1 is a longitudinal sectional view of an amplifying tube ofrectilinear structure embodying the invention;

FIG. 2 is an end view of this tube;

FIGS. 3 and 4 are cross-sectional views of the negative electrode takenon lines AB and CD, respectively, of FIG. 1.;

FIG. 5 is a plan view of the negative electrode;

FIGS. 6 and 7 are cross-sectional views of other em- 15 bodiments ofnegativeelec'trodes of tubes embodying the invention;

FIG. 8 is across-sectional view of a backward travelling wave oscillatortube of circular structure embodying the invention;

FIGS. 9 and 10 are views of variants of the terminal parts of the tubeshown in FIG. 1, and

FIGS. 11 to 15 are diagrammatic views of other examples of -tubesembodying the invention.

'The tube shown in FIG. 1 comprises, in the known manner, a metallicenvelope 4, a delay line 5 with its input 6, its output 7 and itsattenuation 8. A source of current 26 brings this delay line, theenvelope 4 andthe collector 9 to a positive potential relative to thecathode 1. Thelatter'is supported by thenegative electrode "27 which isparallelto the-delay line 5. The cathode is heated by the circuit 28 Isupplied from a source of energy, not shownin the drawing, connected for"exampie to the electrode 27. The interaction space bounded -bytheelectrodes 5 and 27 is traversed by'a magnetic held, the lines offorce of which are perpendicular to the plane of the figure; this fieldis generated by magnets 29 seen in FIG. 2. According to the invention,the

negative electrode 27 consists of twoelements 2 and 3,

cross-sectional views of which are shown in FIGS. 3 40 .and 4respectively.

The element -2 of the negative electrode adjacent the cathode comprisesa fiat surface disposed between two longitudinal raisededges or flanges12 (FIG. 3). This surface may be-covered by a layer 10 of material pos-.sessing ahigh coefiicient of secondary emission. It is obvious that theelement 2 may be composed entirely of a homogeneous material having ahigh coefi'icient of secondary emission; in this case, the layer 10 isno longer necessary.

The-element'3,-'shown in FIG. 4, is provided with longitudinal traps 13,known per so (see U.S. application Serial No. 429,346, filed by Reverdinon May 12, 1954); these traps are preferably disposed in a directionparallel with the-direction of the beam. FIG. 5, which is aplan view ofthe electrode 27, clearlyshows the respective positions of the elements2and 3.

A'tube of this kind operates in the following manner: The electrons fromtheca'thode 1 which. impinge upon thesurfacelO of theelement 2, cause ahigh secondary emission. When the beam interacts with the travellingwave propagated along the delay line 5, a part of the electrons -isabsorbed" by the delay line. The electrons thereby eliminated arereplaced in the beam by the sec- .ondary electrons emitted by thesurface 10. The num- -ber of-electrons interacting with the wavetherefore remains substantially constant between the cathode and .thecollector andthe power output of the tube is im- .proved. Anysecondaryemission caused by the impact -of electrons on the element 6 of thenegativeelectrode, however would produce a high current between thenega- "tive electrode and the collector, whereas the length of the pathofthe secondary electrons would be too short to a1- similarly circularstructure, are known per se.

3 low their interaction with the wave to contribute to amplification.This harmful emission is avoided owing to the presence of the traps 13.The electrons impinging on the element 3 are captured by these traps andthe secondary electrons are incapable of escaping therefrom.

Good results have been obtained by giving the element 2 a length whichis five times that of the element 3. Indeed, it is fairly advantageousto make the absorbing element 3 fairly long so that it is capable ofabsorbing, without causing secondary emission, as many as possible ofthe electrons that have not imparted sufficient energy to the wavepropagated in the line. On the other hand, the element 2 must also belong enough to cause all the useful secondary electrons to participatein the interaction. The aforementioned proportion of 5:1 as between thelength of elements 2 and 3- satisfies these contradictory conditions.

The start of the absorbent element 3 must not be too near the end of thedelay line and preferably the absorbent element should be prolongedbeyond this end. In this way the current of the collector is diminished.Indeed, a large part of the electronic current is absorbed by theelement 3; the losses due to the current of the collector are thusreduced.

The layer may be obtained in various ways, for example a thin layer ofmetal such as platinum deposited electrolytically, or of a certain alloyfor example copperberyllium. Certain oxides, and in particular alkalineoxides, are evenmore efiective than metals. But these oxides aregenerally difficult to deposit in the form of a thin layer capable ofstrongly adhering to a metallic surface. They require a rough or porousmetallic surface. For obtaining this rough or porous surface, oneprocess consists in mechanically or chemically treating the metallicsurface of the electrode 2; for example, a suitable molten metal may besprayed on this surface by means of, for example, a Schoop gun.

According to the variant shown in FIG. 6, the layer 10 having a highcoefiicient of secondary emission does not extend over the entire widthof the electrode 2, shown in FIG. 2, but terminates at a certaindistance b from the flanges 12 thereby defining with the latter twobands 16. The width b of these bands 16 is chosen to be about equal tothe height a of these flanges. In order to eliminate any secondaryemission on the bands 16, a slit 17 may be machined into each of thelatter (FIG. 7) along the length of electrode 2; several similarparallel slits could of course be provided in each band. These slitsabsorb the primary electrons which impinge on the negative electrode inthe same way as the traps 13 of the element 3 and thus traps areprovided along at least the side portions of the negative electrode overits entire length. Thus, the secondary emission occurs in a zone whosewidth is precisely determined; furthermore, the beam does not extendinto the lateral regions of the interaction space, where the highfrequency fields are often too weak, to cause an appreciableinteraction.

There is shown in FIG. 8 a backward travelling wave oscillator ofcircular form embodying the invention. The differences between thisoscillator and an amplifier of These differences lie in the dispositionof the output 107 adjacent the cathode 101 emitting the electron beamand in the disposition of the attenuation 108 on the delay line 105adjacent the collector 109. The interaction space is traversed by amagnetic field the lines of force of which are perpendicular to theplane of the figure, as in the case of the tube shown in FIG. 1. Thesame two elements 2 and 3, referenced 102 and 103, are also in thenegativeelectrode, together with the layer 110.

It is possible in this embodiment not to prolong the absorbent element103 beyond the end of the delay line 105. Indeed, the last part of thisdelay line is covered by the attenuation 108 and does not participate inthe interaction mechanism. The element 103 is thus, in cffeet, alreadyprolonged beyond the effective end of the line, i.e. from the start ofthe part covered with the attenuation 108. l

It is obvious that it is of interest to make the element 3 as long aspossible so as to reduce the losses of the collector. However, thisshould not be done to the detriment of the element 2 for the reasonsexplained above. It would be possible, at the cost of lengthening thetube, to prolong the element 3 beyond the output end of the delay line5. To avoid so increasing the length of the tube the element 3 may begiven an incurved form as shown at 203 in the tube illustrated in FIG.9; it is disposed round the element 215 and is integral with theelectrode 227. The electron paths incurve under the effect of themagnetic field and follow the annular passageway 228 between the element204 and the element 203. The electrons, which are not captured by theelectrode 203, rejoin the collector 209 which is brought to a highpositive potential.

Similarly, it may be advantageous, as shown in FIG. 10, to increase thelength of the part of the element 202 situated between the cathode 201and the point facing the entrance 6 of the delay line 205. In FIG. 10,the element 202, which has a high coefiicient of secondary emission onaccount of the layer 210*, permits utilization of a cathode 201 having aweak emission. The electronic current at the entrance of the interactionspace may be rendered sufiiciently intense by the addition of secondaryelectrons. If the negative electrode is rectilinear, the necessity tolengthen the tube once more arises. To avoid this, the tube may beconstructed as shown in FIG. 10 with the part 229 of the electrode 202wound round an element 214 integral with the electrode 227. Under theeffect of the electric and magnetic fields, the electrons in the space230 between the electrode 204 and the element 214 move in a circularpath which conducts them to the entrance of the interaction space.

FIGS. 11 to 15 illustrate some variants of tube embodying the invention.In these figures the electrodes in question are illustrateddiagrammatically together with their supply sources.

1 The tube diagrammatically illustrated in FIG. 11 comprises threenegative electrodes 320, 302 and 303 facing the anode 304 and the delayline 305 both of which latter are brought to the same potential.Electrode 320 supports the cathode 301. A layer 310 having a highsecondary emission is disposed on the surface of the electrode 320between the cathode 301 and the entrance of the interaction space at306. A second electrode 302 follows the electrode 320. The electrode 302faces the delay line 305 and it is brought to a negative potentialrelative to that of the cathode 301.

In order to maintain the same electric field in both the space 304301and the space 305-302, the electrode 302 is situated at a distance fromthe electrode 305 greater than the distance between the electrode 320and the electrode 304. The electrode 302 also comprises, over the partof its surface between the attenuation 308 and the output 307 of thedelay line 305, a layer 310' having a high secondary emission.

The secondary electrons emitted by the layer 310 are added to theprimary electrons emitted by the cathode 301. The totality of theseprimary and secondary electrons forms the beam that enters in theinteraction space.

As the exchange of energy from the wave to the electrons is particularlyintense in the vicinity of the portion of the line comprised between theattenuation 308 and the output 307, it is advantageous to provide, inthis region, a layer 310', having a high emission coefiicient, whosesecondary emission augments the intensity of the beam. This emission isfavored by the low'potential of the layer 310' which permits anenergetic acceleration of the secondary electrons, the potentialdifference between the layer 310' and the collector being high.

The electrode 303 is in alignment with the electrode "302 andis'situated at the right of the output 307 ro'fthe delay line 305, asviewed in FIG. 11. This electrode is -brought to a potential higher thanthat of the electrode "302. Consequently, the potential diifere'ncebetween the source of potential with connections thereto is shown In thetube shown in FIG. 12, the negative electrode 27 of the FIG. 1 isreplaced by a system of three electrodes 420, 421 and 422. The electrode420 supports the cathode40l1 and a layer 410 having a high secondaryemission. The electrode420 is situated facing the anode 404 at the leftof the interaction space, as viewed in FIG. 12. The electrode 421 facesthe line 405 in the region comprised betweenthe entrance 406 and thefirst end of the attenuation 408.

The electrode 422 extends along the last portion of the interactionspace between attenuation 408 and the end 407 of the interaction space.The electrode 422 supports a second layer 410 having a high emissioncoefficient and the absorbent element 403. The electrode 421 is at thehighest negative potential. After this, in the order of increasingpotentials, are the electrode -422 and the electrode 420 both of whichare at substantially equal potentials. In order to maintain the fieldsubstantially constant in the spaces 420-404 and 421405, the electrode421 is placed at a distance from the delay-line 405 which isgreater thanthat of the electrode 420 therefrom. The electrodes 420 and 422 aresubstantially in alignment. The electrode 422 is a little more negativethan electrode '420 and the electric field existing in the spaces420-404 and 421405 is a little weaker than that in the space 422- 405.Thus the secondary electrons in the space 422-405 are subjected to agreateraccelerating voltage, which to a certain extent favors thesecondary emission. Such an arrangement permits defining with precisionthe portion of the interaction space where the beam is partiallysupplied by the secondary emission.

In the following description of embodiments shown in FIGURES 13, 14 and15, there are many similarities to the apparatus shown in FIGURE 11 andlike reference numerals are used to denote like parts in each of thesefigures.

In the tubeshowvn in FIG. 13, the'absorbent electrode 303 is Whollyinside the interaction space. The electrode 303 has been moved nearerthe delay line 305. The distance of the electrodes 320, 302 and 303 andtheir respective potentials have been so selected that the field between"303 and 305 is substantially equal to the field between 302 and 305.

In the tube shown in FIG. 14, the'electrode 303 has been split upintotwo parts, the split being situated facing the end 307 of the delayline. The first part 303a is united with the electrode 302 andthe'second part 3031) is at the 'same level but is brought to a lessnegative potential than'uhe electrode 302.

The potentials of the various electrodes are the same as those of thetube shown in FIG. 13. Hence, the elec trons that are capable ofattaining the electrode 303a are absorbed'there by an electrode broughtto a very negative potential. The electrons which attain the elec-"trode 303b are absorbed there by an electrode brought to a negativepotentialwhichisless negative orlower in absolute value.

In thetube shown-in FIG. -15, the electrode 303b of FIG.- 14 is replacedby a series of absorbent sections 303b, 3031) and 303b', broughtrespectively to differnt fixed ngative' potentials, relatively to thecathode 301. Said potentials are all the higher as these electrodes arenearer to the collector. In this way, the electrons are always capturedby an electrode 3113b brought to a potential which is only slightlyhigher than the potential corresponding to their kinetic energy. It isobvious that the electrons 'c'aptured'by the electrode 303 b have akinetic Theresultant secondary emission is always weak, and the outputenergy of the tube is improved.

1. In a travelling-wave magnetron tube of the type comprising a delayline having two extremities and an electrode system electricallynegative with respect to said means for producinga magnetic'field havingits lines of force directed through said space perpendicular tothe linesof force of said electrostatic field and to thedirection of said primarybeam, and electron collecting means disposed at said outlet: the surfaceof said negative electrode systemfacing said delay line having at leastone first portion near said beam producing means with a high secondaryomission factor, and at least one second portion disposed near saidelectron collecting means, and provided with means for absorbingelectrons impinging thereupon without releasing secondary electronstherefrom.

2. Tube as claimed in claim 1, wherein said first portion extends overthe entire length between said beam producing means and said secondportion.

3. Tube as claimed in claim 1, wherein said first portion extends overthe length between said beam producing means and the extremity of saiddelay line defining said inlet.

4. Tube as claimed in claim 1, wherein said delay line is provided withattenuating means in a region intermediate the extremities thereof, saidattenuated region having two extremities, said first portion extendingfrom the point of said negative electrode system facing the extremity-ofsaid attenuated region remote from said beam producing means to saidsecond portion.

5. Tube as claimed in claim 1, wherein the portion of said negativeelectrode system between said beam producing means and the pointdefining said inlet is curved.

6. Tube as claimed in claim 1, wherein the part of said second portionextending beyond the pointdefining said outlet is curved.

7. Tube as claimed in claim 1, wherein said secondportion is between twobands, said bands extending in the direction of said beam and having asmaller secondary emission factor than said first portion.

8. Tube as claimed in claim 7, wherein longitudinal grooves are providedin said bands.

9. Tube as claimed in claim 1, wherein said negative electrode systemcomprises two longitudinal edge portions extending in the direction ofsaid beam perpendicular to the surface of said electrode system andbounding said negative electrode system on both sides thereof.

10. Tube as claimed in claim 1, wherein said negative electrode systemcomprises a single electrode.

11. Tube as claimed in claim 1, wherein said negative electrode systemcomprises several separate electrodes.

12. Tube as claimed in claim l1,wherein said separate electrodes aredisposed "at different distances from'said delay line.

13. Tubeas claimed in claim 11, wherein means are provided for biasingsaid separate electrodeswith difierent negative potentials with respectto said delay line.

"14."Ina travelling wave tube of the type comprising a delay line and anelectrode system parallel to and facing said delay line and definingtherewith a wave and electron interaction space having inlet and outletends, means for producing and directing a beam of electrons through saidspace from said inlet to said outlet in a direction parallel to saiddelay line and electrode system including means for applying a potentialdifference between said delay line and system to establish a transverseelectric field in said space and means for establishing in said space amagnetic field having its lines of force perpendicular to the lines offorce of said electric field: said electrode system including electrodemeans along a first-portion of said inner action space for producingsecondary emission in response to impinging thereon of electrons fromsaid beam and a negative electrode means comprising at least a portionadjacent said outlet provided with means for absorbing electronsimpinging thereon without releasing secondary electrons therefrom.

15. In a travelling wave tube of the type comprising a delay line and anelectrode system parallel to and facing said delay line and definingtherewith a wave and electron interaction space, means for producing anddirecting a beam of electrons through said space from one end thereof tothe other end in a direction generally parallel to said delay line andelectrode system including means for applying a potential diiferencebetween said delay line and said system to establish an electric fieldin said space transversely with respect to said direction and means forestablishing in said space a magnetic field having its lines of forceperpendicular to the lines of force of said electric field and to thedirection of said beam, a negative electrode included in said systemcomprising at least a portion adjacent said outlet provided withelongated traps with their lengths extending generally in the directionof the beam for absorbing beam electrons en- 'tering said traps withoutreleasing secondary electrons therefrom.

16. In an electron tube having a source of primary electrons, a pair ofspaced electrodes between which said electrons are propagated in theform of a beam, means for guiding said electrons to move past one ofsaid electrodes, said guiding means including means for establishingcrossed electric and magnetic fields between said electrodes, said oneelectrode being located so that it may be struck by some of the beamelectrons and including elongated traps with their lengths extendinggenerally in the direction of the beam for receiving electrons impingingon said one electrode and entrapping secondary electrons to reducesecondary emission from said one electrode.

17. An electron tube according to claim 16, wherein said traps include aplurality of grooves extending gen- .erally in the direction of electronbeam propagation.

18. An electron tube according to claim 16, wherein said one electrodeincludes elongated traps along substantially its entire length, saidtraps being elongated generally in the direction of beam propagation.

19. An electron tube according to claim 18, wherein said traps are aplurality of parallel slits in said one electrode.

20. In a travelling wave tube of the type comprising a delay line and anelectrode system parallel to and facing said delay line, means forproducing and directing a beam of electrons through the space defined bysaid delay line and electrode system and in the direction generallyparallel thereto, said beam directing means including means forestablishing a transverse electric field in said space and means forestablishing in said space a magnetic field having its lines of forceperpendicular to both the lines of force of said electric field and tothe direction of said beam, said electrode system including an electrodeat a negative potential relative to said delay line and comprising atleast a portion provided with elongated traps with their lengthsextending generally in the direction of the beam for absorbing electronsimpinging thereon and minimizing the release of secondary electronstherefrom. v

21. A travelling wave tube according to claim 20, wherein said trapsinclude a plurality of grooves in said negative electrode, said groovesextending generally in the direction of said electron beam.

22. In an electron tube having a source of primary electrons, a pair ofspaced electrodes, means for propagating said electrons generally in theform of a beam in said space and generally in a direction along thesurface of one of said electrodes, means for establishing crossedelectric and magnetic fields between said electrodes, each field beinggenerally perpendicular to the path of said beam between saidelectrodes, for guiding said electrons to move generally in saiddirection, said one electrode being so located that it may be struck bysome of the beam electrons and including traps for reducing thesecondary emission from said electrode caused by beam electronsimpinging thereon, said traps including grooves in said one electrodehaving a component of their length extending in the direction of saidbeam.

23. In a travelling wave tube of the type comprising a delay line and anelectrode system parallel to and facing said delay line, means forproducing and directing a beam of electrons through the space betweensaid delay line and said electrode system, said means including meansfor establishing crossed electric and magnetic fields each generallyperpendicular to the path of the electron beam, said electrode systemincluding an electrode in a position where it may be impinged by somebeam electrons and comprising at least a port-ion provided with trapsfor reducing the secondary emission therefrom due to impingement of beamelectrons thereon, said traps being defined by elongated electricallyconducting structures on said one electrode, said structures each havinga component of their length extending generally in the direction of saidelectron beam in said space.

24. Electron tube apparatus according to claim 16, wherein said meansfor establishing crossed electric and magnetic fields between saidelectrodes includes means for maintaining said one electrodeelectrically negative with respect to the other of said pair of spacedelectrodes.

25. Electron tube apparatus according to claim 24, wherein saidelectrodes have circularly arcuate form defining an arcuate space forpropagation of the electron beam.

26. Electron tube apparatus according to claim 25, wherein said otherelectrode of said pair of spaced electrodes is a delay line structure.

References Cited in the file of this patent UNITED STATES PATENTS2,312,723 Llewellyn Mar. 2, 1943 2,401,777 Shepherd July 11, 19462,520,603 Linder Aug. 29, 1950 2,547,142 Shepherd Apr. 3, 1951 2,582,185Willshaw Jan. 8, 1952 2,607,904 Lerbs i Aug. 19, 1952 2,613,335 Fremlinet a] Oct. 7, 1952 2,680,209 Veronda June 1, 1954 2,694,783 Charles Nov.16, 1954 2,695,929 Reverdin Nov. 30, 1954 2,704,350 Lerbs Mar. .15, 19552,741,718 Wang Apr. 10, 1956 2,853,641 Webber Sept. 23, 1958 2,861,212Lerbs Nov. 18, 1958 FOREIGN PATENTS 1,081,937 France June 16, 1954

